Accurate small-spot spectrometry instrument

ABSTRACT

The invention is a method and apparatus for determining characteristics of a sample. The system and method provide for detecting a monitor beam reflected off a mirror, where the monitor beam corresponds to the intensity of light incident upon the sample. The system and method also provide for detecting a measurement beam, where the measurement beam has been reflected off the sample being characterized. Both the monitor beam and the measurement beam are transmitted through the same transmission path, and detected by the same detector. Thus, potential sources of variations between the monitor beam and the measurement beam which are not due to the characteristics of the sample are minimized. Reflectivity information for the sample can be determined by comparing data corresponding to the measurement beam relative to data corresponding the monitor beam.

RELATED APPLICATION

[0001] The present application claims the benefit of U.S. ProvisionalApplication SerIAL No. 60/337,678, filed Nov. 9, 2001, titled ACCURATESMALL-SPOT SPECTROMETRY INSTRUMENT which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates generally to metrology instruments,and more particularly to spectrometry instruments that measure thereflectivity of a sample.

BACKGROUND

[0003] In semiconductor manufacturing and other fields it is desirableto make quantitative measurements of a sample's reflectivity propertiesover very small, selectable areas and over as broad a band ofwavelengths as possible. Instruments for making these measurementstypically incorporate microscope-like objective lenses for focusinglight on the sample. One type of illumination source is a Xenon arclamp. In order to scan to various positions on the sample of interest aportion of the optics can be moved with respect to a stationary portionof the optics, or the sample can be moved with respect to at least someportion of the optics, or both.

[0004] A common issue with such instruments is guaranteeing thestability of the light which is incident upon the sample, or at leastknowing the spectral intensity of the incident light, so that thedetected light reflected from the sample can be evaluated relative tothe intensity of the light incident on the sample. Since reflectivity isdefined as the ratio of the intensity of light reflected off the samplerelative to the intensity of light incident upon the sample, accuratereflectivity measurements depend knowing the incident light intensity.

[0005] There are several factors that can make it difficult to determinethe intensity of the light upon the sample. One factor is that thecharacteristics of most sources of light change with time, and thus theintensity of the incident light can vary with time. Another factor isthat where there is relative motion of the illumination source and therest of the optics, either via (nearly) collimated light paths oroptical fibers, there can be changes in the transmission efficiency ofilluminating light as a function the scan position, or scan state. Herescan state includes the history of previous scan positions. This isimportant, for example, with some architectures using a fiber totransmit light from the light source. One prior system is shown in USpatent application, publication US 2002/0021441 A1 (SMALL SPOTSPECTROMETRY INSTRUMENT WITH REDUCED POLARIZATION) and in PCTapplication, international publication number WO 00/57127 (METHOD ANDAPPARATUS FOR WAFER METROLOGY); both of these references areincorporated herein by reference in their entirety.

[0006]FIG. 1 shows another type of prior system 100. The system 100includes a light source 102 and a transmission means 104 for lightgenerated by the light source 102. The light transmitted through thetransmission means 104 is then transmitted through a collimating lens108, and leaves the collimating lens as light beam 106. The light beamis then incident upon a beam splitter 110. A first beam 140 istransmitted from the beam splitter through a lens 144 and then through aplate 146 having pinhole to receive the first beam 140. The first beam140 is then transmitted through a transmission means 148 and received bya detector 150. In response to the light received, the detector 150generates a monitor signal corresponding to the received light. Thismonitor signal from the detector 150 is received by a processor 160which analyzes the monitor signal and uses it relative to a signalgenerated by detector 130.

[0007] In addition to the beam 140 being transmitted through the beamsplitter 110, beam 112 is also reflected from the beam splitter 110through an objective lens 114 and onto a spot 118 on a sample 116 beinganalyzed. Some portion 113 of the light 112 is reflected off the sampleand back through the objective lens 114. This light 113 is furthertransmitted through the beam splitter 110 and off a turn mirror 122 andthrough a lens 124. The resulting light beam is then transmitted througha pinhole in a plate 126 and into a transmission means 128. The lighttransmitted through the transmission means 128 is received by thedetector 130. In response to receiving this light the detector 130generates a sample signal which corresponds to the received light. Thissample signal is received by the processor 160 where it is analyzedrelative to the monitor signal received from the detector 150.

[0008] The fact that prior systems provide for transmitting a monitorbeam 140 and a measurement beam 113 through different transmission pathsand provide for using different detectors (150 and 130) for detectingthe monitor light beam and the measurement light beam introduces anumber of potential sources which could generate variations in themonitor signal relative to the measurement signal which are not relatedto the reflective properties of the sample. What is needed is a systemwhich reduces possible sources of extrinsic variations in the monitorbeam relative to the measurement beam.

SUMMARY OF THE INVENTION

[0009] One embodiment herein provides a system for measuringcharacteristics of a sample. This system includes a light source forgenerating a beam of light which is directed toward a sample, and amirror which can be moved between a first position and a secondposition, wherein in the first position the mirror is positioned betweenthe light source and the sample such that light generated by the lightsource is reflected off the mirror and transmitted through a first path.The first path consists of a reflection path. In the second position themirror is positioned such that it is not between the light source andthe sample, and light generated by the light source is reflected off thesample and transmitted through the reflection path. This system alsoprovides a detector coupled to the reflection path which generates amonitor signal in response to receiving light reflected from the mirror,and generates a measurement signal in response to light reflected fromthe sample.

[0010] Another embodiment includes a method for determiningcharacteristics of a sample in a system having a light source, and amovable mirror. The method includes generating a light beam anddirecting the light beam toward the sample, and positioning the movablemirror such that it is in a first position, where it reflects the lightbeam along a first path, wherein the first path consists of a reflectionpath. The method also includes generating a monitor signal whichcorresponds to the light reflected from the mirror, and positioning themovable mirror such that it is in a second position, where it does notreflect the light beam which is directed toward the sample, wherein thelight beam which is directed toward the sample is reflected off thesample along the reflection path. The method of this embodiment alsoincludes generating a measurement signal which corresponds to the lightreflected from the sample, and analyzing the measurement signal relativeto the monitor signal to determine properties of the sample.

[0011] Another embodiment includes a system for measuringcharacteristics of samples. The system includes a light source forgenerating a beam of light, and beam splitter for directing the beam oflight toward a sample. The system also includes a lens disposed betweenthe beam splitter and the sample for focusing the beam of light on thesample, such that a measurement beam of light is reflected off thesample, wherein after the measurement beam is reflected off the sample,it is transmitted through a reflection path. The system includes adetector positioned to receive light transmitted through the reflectionpath wherein in response to receiving light transmitted through thereflection path the detector generates a signal corresponding to thelight transmitted through the reflection path, and a mirror which can bemoved between a first position and a second position, wherein in thefirst position the mirror is positioned between the beam splitter andthe sample, such that light directed by the beam splitter toward thesample is incident upon the mirror and reflected through a first path,wherein the first path consists of the reflection path, wherein in thesecond position the mirror is positioned such that light directed by thebeam splitter toward the sample is reflected off the sample along thereflection path. The system further includes a processor coupled to thedetector which uses a first signal generated by the detector in responseto receiving light reflected from the mirror and a second signalgenerated by the detector in response to light reflected from thesample, to determine characteristics of the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic view illustrating a prior art spectrometersystem.

[0013]FIG. 2 is a schematic view of an embodiment of the presentinvention in a first state.

[0014]FIG. 3 is a schematic view of an embodiment of the presentinvention in a second state.

[0015]FIG. 4 is top view of an embodiment of a movable mirror and itsmounting.

[0016]FIG. 5 is a side view of an embodiment of a movable mirror andcomponents which enable its motion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017]FIG. 2 illustrates one embodiment of a multiplexed spectrometersystem 200 of the present invention in a first state. The system 200includes a light source 102 coupled to a transmission means 104, such asan optic fiber. The light transmitted through the transmission means 104is then transmitted through a collimating lens 108 which transmits abeam of light 106. The beam of light 106 is then incident upon a beamsplitter 110. An absorber 202 absorbs light passing through beamsplitter 110. Illumination beam 206 reflects from the beam splitter 110toward a sample 116. A movable mirror 204 is shown in a monitorposition. In the first position, the mirror 204 is positioned betweenthe beam splitter 110 and the sample 116, such that it reflects beam 208through a reflection path. As part of this reflection path, thereflected beam 208 passes through the beam splitter 110, reflects off anoptional turn mirror 122, and passes through focusing optics 224, suchas a lens, onto a pinhole in a plate 226. Reflected beam 208 then passesthrough a transmission means 228, such as an optic fiber and is thenreceived by a detector 230. Typically, the detector 230 will be aspectrometer, which detects the intensity of different wavelengths oflight. One type of spectrometer includes an optical element forangularly dispersing a light beam as a function of wavelength. Thisdispersed light is then measured by an array of detector elements. Withsystem 200 in the first state, detector 230 generates a monitor signalcorresponding to the intensities at different wavelengths of lightreflected from movable mirror 204, and not from sample 116. Theprocessor 160 then uses the monitor signal as an indication of intensityin the illuminating light, based on the assumption that mirror 204 doesnot change with time, i.e., between calibrations, as discussed below.

[0018]FIG. 3 shows the system 200 in a second state where the movablemirror 204 is in second position. As shown in the second position, themirror 204 is not between the beam splitter 110 and the sample 116. Asin FIG. 2, the light beam 106 is incident upon the beam splitter 110,and the absorber 202 absorbs light passing through the beam splitter110. Again illumination beam 206 reflects from beam splitter toward thesample 116. In this second state, beam 206 passes through an objectivelens 114 and is incident upon at least a small spot 118 on the sample116. Reflected beam 208 is then transmitted through the reflection path,as described above, and received by detector 230. The detector 230 thengenerates a sample signal indicative of the reflectance of spot 118.

[0019] Apparatus 200 is generally for measuring the reflectivity of asample. Reflectance is defined as the ratio of intensities incident uponand reflected from the sample. System 200 must be calibrated in order tomeasure reflectance. Calibration finds the relationship between measuredsignals and samples with known calibration reflectivities. Calibrationmay include the steps of estimating the calibration reflectivities.Calibration preferably allows for changes to system 200 over relativelylong time scales, e.g., months. In the discussion that follows, allsignals from spectrometer 230 are preferably dark corrected bysubtracting signals collected with no light, e.g., from transmissionmeans 104, as is known in the art. Further, the signals mayalternatively be additionally corrected for scattering of light withinspectrometer 230, as is also known in the art. A sample signal iscollected with mirror 204 retracted so that sample 116 reflectsreflected beam 208, as in FIG. 3, and a monitor signal collected withmirror 204 reflecting reflected beam 208, as shown in FIG. 2.

[0020] In an embodiment of system 200, and in another embodiment asdescribed in PCT application no. PCT/US00/07709 entitled APPARATUS FORWAFER METROLOGY (which is hereby incorporated by reference in itsentirety) the system 200, at any desired time when system is in use,locate spot 118 on a reference sample whose reflectivity does not changewith time, and collect a reference measurement signal. The referencemeasurement signal has a corresponding reference monitor signal. In someembodiments, the reference measurement and monitor signals are collectedat a time as close as practical to the time for collecting the samplemeasurement and monitor, e.g., with a time difference of less than aminute.

[0021] The estimated value of the reflectance is given by

R(λ,r)˜=S(1, λ,r)S(2, λ, r ₀)/[F1(λ,r)S(2, λ,r)S(1, λ, r₀)]−F0(λ,r)/F1(λ,r)  Eq 1

[0022] where S(

, λ,r) is the measurement signal from the sample at location r whenmirror 204 is in its second state, S(

, λ, r) is the corresponding monitor signal at the same location (e.g.with the optics in the same position as when the measurement signal forlocation r is generated) when mirror 204 is in its first state. S(

, λ, r₀) is the measurement signal from the reference reflector at r₀acquired at a proximate time, S(

, λ, r₀) is the corresponding reference monitor signal, F0(λ,r) andF1(λ,r) are first and second calibration functions, λ is wavelength, rspecifies the position of spot 118 relative to sample 116, and r₀ is therelative position of the reference sample. The calibration functions arethe result of minimizing $\begin{matrix}{\sum\limits_{n = 1}^{N}\left( {\frac{{S\left( {n,1,\lambda,r} \right)}{S\left( {n,2,\lambda,r_{0}} \right)}}{{S\left( {n,2,\lambda,r} \right)}{S\left( {n,1,\lambda,r_{0}} \right)}} - {{F0}\left( {\lambda,r} \right)} - {{{F1}\left( {\lambda,r} \right)}{R_{c}\left( {n,\lambda,r} \right)}}} \right)^{2}} & \text{Eq~~2}\end{matrix}$

[0023] with respect to F0(λ,r) and F1(λ,r). R_(c)(n,λ,r) are reflectanceof calibration samples. The signals S and reflectances R_(c) have anadditional integer index n=1,2, . . . ,N that labels the calibrationsamples. N is the number of calibration samples. In the preferredimplementation, N=2, the reflectance of the calibration samples areknown, and the expression in Eq 2 is minimized with respect to F0(λ,r)and F1(λ,r) separately for each wavelength λ and position r.

[0024] In an alternative embodiment, parameters of one or morecalibration samples, such as the thicknesses of films, are unknown. Inthis case, the following expression is minimized $\begin{matrix}{\sum\limits_{\lambda}{\sum\limits_{n = 1}^{N}\left( {\frac{{S\left( {n,1,\lambda,r} \right)}{S\left( {n,2,\lambda,r_{0}} \right)}}{{S\left( {n,2,\lambda,r} \right)}{S\left( {n,1,\lambda,r_{0}} \right)}} - {{F0}\left( {\lambda,r} \right)} - {{{F1}\left( {\lambda,r} \right)}{R_{c}\left( {n,\lambda,r} \right)}}} \right)^{2}}} & \text{Eq~~3}\end{matrix}$

[0025] with respect to the unknown parameters of the calibration samplesand F0(λ,r) and F1(λ,r) for all wavelengths simultaneously. Theminimization is repeated for each position r.

[0026] In alternative embodiments, Eq 2 may include terms withadditional calibration functions multiplied by powers of R_(c). Eqs 1and 2 thus apply to the linear, or first order calibration, and higherorder calibrations are possible. In yet alternative embodiments, thereference reflector and measurements associated with it may be left out.In such cases, movable mirror 204 serves as the reference reflector.Alternative version of Eqs. 1 and 2 would yield measured reflectivityand calibration functions.

[0027] As seen above, reflectance is generally related to the ratiobetween the sample and monitor signals. Because both sample measurementsignal and monitor signal come from light in the reflection path, indifferent states of the instrument 200, changes in this ratio are due tothe reflectivity changes of the sample 116 and mirror 204 only, and notpotential variations caused by utilizing different reflection pathsand/or different detectors for the monitor and measurement signals. Thereflection path is such that the light reflected off the mirror to thedetector includes only elements which the light reflected from thesample to the detector will travel through. Thus, there are no elementsin the reflection path from the mirror to the detector which are notincluded in the reflection path from the sample to the detector.

[0028] Thus, in prior systems there is a much higher likelihood that theratio between the monitor signal and the measurement signals depends notonly on the reflectivity characteristics of the sample, which arepresumed unknown, but also on the effects of the different reflection ortransmission paths, and different characteristics of differentdetectors. For example, referring to FIG. 1, a spec of dust on lens 144would affect the ratio measurement/monitor ratio. However, referring toFIGS. 2 and 3, a spec of dust on lens 224 will affect the measurementand monitor signals equally so that their ratio will remain constant.Similarly, different changes in temperature of detectors 130 and 150 inFIG. 1 are likely to change their efficiencies differently, and thusaffect the measurement/monitor ratio. However, in the present inventionthere is only one detector, so the ratio will not change as long as thetemperature and efficiency of the detector do not change over the shorttime required to sample the two signals. Thus, the present invention canprovide for enhanced measurement accuracy of the reflectioncharacteristics of the sample.

[0029] Movable mirror 204 may be implemented as shown in FIGS. 4 and 5.Viewed from above, as in FIG. 4, mirror 302 is held with bracket 304which is allowed to rotate about axel 306 to positions 305 a and 305 bby bearings 308. Support 310 holds the bearings in a fixed relation tothe rest of the optics in system 200, beam splitter 110, as shown inFIGS. 2, 3, and 5. Motor 312 turns axel 306, and consequently bracket304. Motor 312 preferably has hard stops associated with locations 305 aand 305 b, to allow these positions to be highly reproducible. Twospaced bearings 308 are preferred to constrain motion of bracket 304 tobe in a plane so that mirror 302 is always perpendicular to the opticalaxis of associated with path 206. Many alternative embodiments arepossible. For example, mirror 302 may be allowed to rotate 360° aboutaxel 306. Mirror 302 may rotate continuously, with the preciseacquisitions times for measurement and monitor signals synchronized withthe rotation. In yet alternative embodiments, both the sample andmonitor signals may be sums of over alternating sample and monitorsignal portions. This allows for, e.g., variations in lamp intensity ata faster rate.

[0030] While the present invention has been described in terms of theembodiments discussed above, those skilled in the art will recognizethat the present invention may be practiced with modification to theabove described embodiment and still be and within the spirit and scopeof the appended claims. For example, one alternative embodiment couldprovide for positioning the movable mirror between the objective lensand the sample. Thus, the specifications and figures herein are to beregarded in an illustrative rather than a restrictive sense. Further,even though only certain embodiments have been described in detail,those having ordinary skill in the art will certainly understand thatmany other modifications are possible without departing from theteachings herein. All such modifications are intended to be encompassedwithin the following claims.

What is claimed is:
 1. A system for measuring characteristics of asample, including: a light source for generating a beam of light whichis directed toward a sample; a mirror which can be moved between a firstposition and a second position, wherein in the first position the mirroris positioned between the light source and the sample such that lightgenerated by the light source is reflected off the mirror andtransmitted through a first path, wherein the first path consists of areflection path, wherein in the second position the mirror is positionedsuch that it is not between the light source and the sample, and lightgenerated by the light source is reflected off the sample andtransmitted through the reflection path; and a detector coupled to thereflection path which generates a monitor signal in response toreceiving light reflected from the mirror, and generates a measurementsignal in response to light reflected from the sample.
 2. The system ofclaim 1 further including a processor coupled to the detector, whereinthe processor receives the monitor signal from the detector, andreceives the measurement signal from the detector, and determines thecharacteristics of the sample based on the ratio of the measurementsignal relative to the monitor signal.
 3. The system of claim 1 furtherincluding a beam splitter positioned between the light source and thesample, wherein light from the light source is incident on the beamsplitter and directed from the beam splitter toward the sample.
 4. Thesystem of claim 3 further including an objective lens positioned betweenthe beam splitter and the sample.
 5. The system of claim 3, wherein inthe first position the mirror is positioned between the beam splitterand the sample.
 6. The system of claim 1, wherein the reflection pathincludes: an optic fiber; a beam splitter; and wherein the detector iscoupled to the optic fiber.
 7. The system of claim 1, wherein thedetector is a spectrometer which generates signals corresponding tointensity of light at different wavelengths.
 8. The system of claim 1,wherein the reflection path includes: an optic fiber, and the detectoris coupled to the optic fiber.
 9. A method for determiningcharacteristics of a sample in a system having a light source, and amovable mirror, the method including: generating a light beam anddirecting the light beam toward the sample; positioning the movablemirror such that it is in a first position, where it reflects the lightbeam along a first path, wherein the first path consists of a reflectionpath; generating a monitor signal which corresponds to the lightreflected from the mirror; positioning the movable mirror such that itis in a second position, where it does not reflect the light beam whichis directed toward the sample, wherein the light beam which is directedtoward the sample is reflected off the sample along the reflection path;generating a measurement signal which corresponds to the light reflectedfrom the sample; and analyzing the measurement signal relative to themonitor signal to determine properties of the sample.
 10. The method ofclaim 9 further comprising using a detector to detect the lightreflected off the mirror, and to generate the monitor signal, and usingthe detector to detect the light reflected off the mirror, and togenerate the measurement signal.
 11. An optical system for measuringcharacteristics of samples, including: a light source for generating abeam of light; a beam splitter for directing the beam of light toward asample; a lens disposed between the beam splitter and the sample forfocusing the beam of light on the sample, such that a measurement beamof light is reflected off the sample, wherein after the measurement beamis reflected off the sample, it is transmitted through a reflectionpath; a detector positioned to receive light transmitted through thereflection path wherein in response to receiving light transmittedthrough the reflection path the detector generates a signalcorresponding to the light transmitted through the reflection path; amirror which can be moved between a first position and a secondposition, wherein in the first position the mirror is positioned betweenthe beam splitter and the sample, such that light directed by the beamsplitter toward the sample is incident upon the mirror and reflectedthrough a first path, wherein the first path consists of the reflectionpath, wherein in the second position the mirror is positioned such thatlight directed by the beam splitter toward the sample is reflected offthe sample along the reflection path; and a processor coupled to thedetector which uses a first signal generated by the detector in responseto receiving light reflected from the mirror and a second signalgenerated by the detector in response to light reflected from thesample, to determine characteristics of the sample.